Hi PIet,
Alliance does not have customer level security but does use node level security.
What you could try is define a separate node for this customer. It can be a child node of the normal nodes for your other users accessing this same customer. Then define that as the only node the agent has access to with no dependents.
So for example:
Node BE (user A, B, C with dependents allowed)
Node BE1 (only node access granted to user D with no dependents)
In this scenario, users A,B,C and D are allowed access to node BE1 while user D only has access to node BE1 with the others also having access to node BE.
Another option of course is a single distinct node for this customer with any agent assigned to this node outside of a the normal node tree your company uses.
Node BE (user A, B, C)
Node BE1 (User D)
In this scenario only user D has has access to BE1 and user D has no access to Node BE1.
But again, customer level access security is not a security function in Alliance.
Hope this helps.